The present invention relates to a metal seal for ball valves and a ball valve comprising such a seal.
It is known in the technical sector relating to the conveying of fluids inside pipes that there exists the need to intercept said pipes using valves that are able to connect two pipe sections and allow/interrupt the flow of fluid through them.
Particularly important in this connection are ball valves, e.g., valves in which the through-flow/interruption in flow can be controlled by a ball which is seated inside the valve body and provided with a through-hole, having a diameter substantially the same as that of the pipes, and which can be rotationally operated between an open position, where the hole is coaxial with the direction of the flow, and a closed position where the hole is transverse to the direction of flow, and in which the seal between the ball and pipe is achieved by means of annular seals arranged between the respective opposite surfaces making relative contact.
In order to achieve sealing of the ball chamber, the valves typically have a ring which is coaxially arranged between the valve body and the ball against which the ring is axially pressed, via elastic means, so as to ensure contact between the sealing surface and the ball itself. Said sealing surface is normally composed of a metal surface of the ring itself which is varyingly made and/or treated or a suitably configured seal which is forced in an axial direction inside a special seat of the ring.
Although performing their function, these seals nevertheless have drawbacks which in practice limit their use particularly in high-temperature and/or high-pressure applications where the metal seals, although better suited, are very costly, both because of the need for a hardening treatment of the contact surfaces of ball and ring acting as seat, and because of the machining operations which are required to adapt and ensure a perfect fit between the contact surfaces of the ball and the two sealing rings.
In addition to the above and as shown in FIGS. 1 to 2b relating to the prior art, it happens also in practice that, during high-pressure applications, the ball and seal tend to be deformed as a result of the pressure exerted by the fluid and these deformations are irregular both in the region of the ball and in the region of the seal, which is no longer able to adapt perfectly to the outer surface of the ball, resulting in zones where there is no contact and therefore no sealing of the valve which leaks.
Examples of the prior art are described in GB 2,206,952 which describes a valve of the floating type, namely with a sealing action provided by the movement of the ball which is displaced against a fixed sealing ring. The sealing ring described consists of a thin elastic plate that flexes like a spring so as to adapt to the surface of the ball.
It follows that, if said surface is not uniform, the relative contact is not precise and leaks occur. Moreover, it is pointed out that the surface of the ring in contact with the ball is formed by a peripheral annular lip 4A and that the incisions 4C which allow deformation of the ring are formed on the surface opposite to the sealing surface.
A further example of the prior art is described in U.S. Pat. No. 4,421,895 in which a control disc rotates against a fixed sealing ring. In this case also the sealing ring is elastic and the sealing surface 30b which reacts against the corresponding surface 28 of the disc does not have milled zones which are instead formed on a rear surface making contact with a retaining ring 36.